What is formed when a flat plain is deposited during the time of flood?

When a flat plain is deposited during a flood, it creates a floodplain, which is an area of land adjacent to a river or stream that is subject to flooding. These floodplains are typically formed by the accumulation of sediment such as silt, sand, and clay, which are carried by the water and deposited as it overflows its banks. Over time, floodplains can become fertile areas that support diverse ecosystems and agriculture.